Plastid transit peptides are N-terminal extensions that facilitate the targeting and translocation of cytosolically synthesized precursor proteins into plastids via a post-translational mechanism (reviewed by Bruce, Biochim. Biophys. Acta 1541:2–21 (2001)). With the sequencing of the entire Arabidopsis genome now completed, it is estimated that more than 3500 different proteins are targeted into the plastids during the life of a typical plant. Developing a model for how all of these targeting sequences function to direct proper targeting has been difficult, since they are highly divergent at the primary sequence level in terms of length, composition, and organization. Secondary and tertiary structural information is only available for a few plastid transit peptides, and the results differ significantly depending on whether the experiments were carried out in an aqueous or membrane-mimetic environment. Thus, no common structural features or properties have been clearly delineated.
The capability to target recombinant proteins to different subcellular compartments in transgenic plants is an important part of plant genetic engineering. For example, many important plant physiological processes take place in plastids including, but not limited to, photosynthesis, fatty acid synthesis, amino acid synthesis, carotenoid biosynthesis, terpenoid biosynthesis, and starch biosynthesis. As such, there is a need for the ability to target recombinant polypeptides to plastids to modulate or alter the physiological processes that occur in the plastids. Additionally, some polypeptides are toxic when expressed recombinantly in the cytoplasm. Because plastids are subcellular compartments, it is possible to target recombinant polypeptides to the plastids to sequester them from the cytoplasm, thus allowing for higher expression levels. Furthermore, expression of recombinant polypeptides in plastids may facilitate isolation of the polypeptide for various applications.